This invention relates to solid waste management, and, more particularly, to sealing disposal sites for landfills, mine tailings piles, and the like.
Material placed on top of solid waste to isolate the waste from the environment is the cover. Covers are primarily used in closing landfills, in stabilizing mine tailings piles, and in isolating other solid, hazardous or radioactive wastes from the environment. They serve to (1) reduce or eliminate infiltration of water which could transport contaminants from the solid waste material into the groundwater system, (2) prevent the release of volatile components or gases that may be present or generated within the solid waste material, and (3) reduce or eliminate dispersal of the solid wastes by natural processes such as wind or erosion, or by activities of animals, plants or humans. A typical cover system design includes one or more of the following components: (1) a compacted clay layer which acts as a low-permeability barrier to prevent infiltration of water or to prevent the release of gases from the solid wastes, (2) a flexible membrane liner or "geotextile" designed to prevent water infiltration into the compacted clay layer, (3) a drainage layer connected to a drainage system which collects and removes infiltrating surface water, (4 ) a biointrusion soil layer including gravel or cobbles to prevent intrusion of burrowing animals and of plant roots, (5) a vegetative layer consisting of top soil for promoting the growth of plants which help control erosion and which remove water from the soil cover by transpiration, and (6) an erosion control layer consisting of rock and plants.
Landfill designs currently require one or more liner systems below the solid waste to collect liquid leachates from the waste pile and to prevent contamination of groundwater. On the other hand, the costs associated with moving large volumes of mine wastes often prohibit installation of a liner or barrier below these wastes, so mine tailings piles are most often stabilized in place by the addition of a cover.
Regulation of landfill designs has largely focused on the performance of the liner and leachate collection systems which are installed below the wastes. EPA design guidelines for hazardous waste landfill covers specify that the cover be no more permeable than the liner system. Thus, standard landfill designs rely on the liner/leachate collection system below the wastes to prevent release of contaminants to the environment, and the cover serves to reduce rather than to prevent the infiltration of water into the wastes. Mine tailings pile covers are, in general, much less regulated than covers for solid waste landfills. The primary exception to this general rule is the case of uranium mine tailings piles which release radon from the radioactive decay of uranium daughter products in the mine wastes. In this application, the cover also serves to prevent the release of radon gas to the atmosphere.
The design lifetime of conventional covers depends upon the material being isolated and on the applicable state and federal laws. Typically, post-closure monitoring and maintenance for municipal and hazardous solid waste landfills is required for a minimum of 30 years (40 CFR) 265.117(a)(1)). Covers for uranium mill tailings are required to control the release of radioactive materials to the environment for 1000 years (40 CFR 192 for inactive sites and 10 CFR 40 Appendix A for active sites). EPA is considering promulgating regulations governing the isolation of non-uranium mine tailings.
Failure of a cover system leads to water infiltration into the solid waste and eventual leakage of the leachate into the environment. Catastrophic failure can occur by any of several methods including: erosion of the cover; internal subsidence of the solid wastes accompanied by rupture of the compacted clay layer; intrusion by plant roots, animals or humans; physical, chemical or biological degradation of the polymer liner; geological activity such as faulting or landsliding; or freezing and thawing cycles. Even without a major failure of the landfill cover, long-term water seepage through the compacted clay layer has been observed in municipal landfills, indicating that compacted clay acts more as a barrier than as a seal. Significant improvements in cover system designs would reduce or eliminate water permeation through the cover by forming a geologically stable seal within the cover that would avert failure of the cover by these mechanisms and/or would be sealed in situ by natural processes.
Accordingly, it is an object of the present invention to form a sealing layer or chemical seal within a cover system by emplacing and compacting a material that will recrystallize to a form that ultimately has a much lower permeability than can be achieved solely by compaction of a cover component material such as soil or clay.
Another object of the present invention is to incorporate a material into the cover layers that can be transported by natural water percolation to the seal zone where it can be incorporated into and be effective to reinforce and stabilize the seal.
An additional object of the present invention is to incorporate a material into the cover design that can repair defects or prevent the deterioration of the cover seal integrity.
One other object of the present invention is to form a seal within a cover system that is enhanced or strengthened by natural processes.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by the practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.